The present invention refers to a device adapted to establish the condition at a surface by measuring of scattered light. This is attained by estimating the interference between the known, scattered light emitted by a reference object, and the unknown, scattered light emitted by the subject in question, which is lighted by the same beam as the reference object. The device is, in the first hand, adapted to measure the roughness of a surface, but may also be utilized for determining velocity, and for a very sensitive and exact registration of weak, scattered light in general.
Devices using laser beams have been used for examining i.a. the smoothness of metal surfaces. By measuring, by means of a photodetector, the intensity of diffusely scattered light, as a function of the angle in relation to a mirror-reflected beam, it is possible to obtain an apprehension of the smoothness of the surface. One advantage with such a method is that it is not necessary to touch the surface.
A disadvantage of this method is that it is rather un-sensitive, especially when fine surfaces are involved, as the intensity of the diffusely scattered light will be low. The method, further, is very sensitive to disturbances caused by incidental light, and it is difficult to determine the angle between the diffuse light, and the reflected beam. On many occasions the angle is small, and it is difficult to maintain the surface completely still, due i.a. to vibrations, normally occuring in a building.